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Title: Molecular mechanism of the Hofmeister series
Plain: Since 1888 it has been known that salts fall in a fixed order, the Hofmeister series, in how strongly they make proteins precipitate. Why ions of the same charge rank this way is still debated.
Precise: Explain the anion ordering of the Hofmeister series (roughly SO4^2- > F- > Cl- > Br- > NO3- > I- > ClO4- > SCN-) in protein salting-out, nonpolar solute solubility and electrolyte surface tension, choosing among bulk water structure making and breaking, direct ion-solute pairing, interfacial adsorption driven by polarizability and partial dehydration, and ion-specific dispersion forces. An answer is a quantitative model with parameters from first principles or independent data that reproduces the ordering and its reversal on positively charged or hydrophobic surfaces.
Settled by: Ion-resolved measurements at protein and model interfaces compared with simulations using polarizable or ab initio potentials across the whole series.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: contested
Status note: Explanations based on bulk water structure lost support in the 2000s in favor of direct ion-interface interactions, but no single quantitative model covers the series (Jungwirth and Cremer, Nature Chemistry 2014, https://doi.org/10.1038/nchem.1899).
Posed since: 1888